Weeds are a tremendous problem for farmers and growers throughout the world. It has been estimated that weeds cause a 10-12% loss of value ($20 billion annually) for agricultural products in the United States (McWhorter, C. G. 1984! WeedSci. 32:850-855). In Florida, losses due to weeds are estimated to be over $100 million per year from vegetable production, alone. About 41% of the cost of plant protection has been estimated to be for the control of weeds. Herbicides are applied to more acres than fungicides and insecticides combined. Weeds act as alternate hosts for insects, fungi, bacteria, and viruses. They affect man, not only by competing with crop plants, but by poisoning range animals, interfering with right-of-waysand roadways, decreasing forest production, and marring landscapes. A particularly notable class of weed is nutsedge.
Nutsedges (Cyperus spp.) comprise a group of commonly occurring weeds that are among the most difficult to control. In the United States and the Caribbean Basin there are three species which are of most consequence, namely C. rotundus (purple nutsedge), C. exculentus (yellow nutsedge), and C. iria (rice flatsedge). Together, these nutsedge species are problematic for virtually every crop grown in Florida. Yield reduction due to nutsedge varies. In agronomic crops, e.g., in maize, yield reduction can be as high as 79%. In horticultural crops, yield reductions can reach 80%. Vegetables, particularly tomato, pumpkin, pepper, and onion, can be affected. Yield reduction in tomato, a very valuable crop in the United States, can be as high as 53%. Nutsedges cannot compete with crops if a dense crop canopy is established, making early season control of these weeds essential.
Purple nutsedge has been called the world's worst weed because of its distribution worldwide and its resistance to control measures (Holm, L. G., D. L. Plucknett, J. V. Pancho, and J. P. Herberger 1977! The World's Worst Weeds: Distribution and Biology. University Press of Hawaii, Honolulu). It is a problem in 52 crops in more than 90 tropical and subtropical countries. It is considered a serious weed in the United States, especially in the warm regions of the southern states from North Carolina to California. It is also considered a principal weed in Puerto Rico.
Yellow nutsedge is more tolerant of cold and drought than purple nutsedge and, therefore, is found over a wider geographic area. It survives as an important weed as far north as Canada and Alaska. Worldwide, yellow nutsedge is not as aggressive and important a weed as purple nutsedge, although, it is a weed of importance in 21 crops in nearly 40 countries of the world and is a serious or principal weed in 15 countries. Yellow nutsedge is a serious or principal weed of sugarcane, maize, potato, cotton, and soybean, chiefly in southern Africa and North America. In some countries, it is listed as one of the three most serious weeds of sugarcane, corn, and potato.
In North America, yellow nutsedge is found in Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Alaska, and all of the contiguous United States except North Dakota. It is a principal weed of sugarbeet and vegetables and is an important weed of maize, potato, soybean, and cotton. In Hawaii, it is found as a principal weed of sugarcane. Yellow nutsedge was one of the most troublesome and costly weeds in the southeastern United States. In cotton production, it ranked as the fifth most important weed.
The weed Kyllinga brevifolia (=Cyperus brevifolius) is an important weed in turfgrass, golf courses, and other situations. Kyllinga is difficult to control with existing chemical herbicides.
Many herbicides have been tested for control of purple and yellow nutsedge. Pereira et al. recently reviewed this research (Pereira, W., G. Crabtree, R. D. William 1987! Weed Technology 1:92-98). Herbicides based on virtually every mode of action have been studied. Examples are: 2, 4-D ("control erratic"), atrazine ("control inconsistent"), linuron ("marginal control"), paraquat ("inconsistent"), alachlor and metolachlor ("control temporary"). Some herbicides that have been used successfully include glyphosate, dichlobenil, EPTC, arsenicals, and soil flumigants. Success or failure of a herbicide treatment depends on such factors as nutsedge growth stage at application, soil moisture and temperature, and addition of adjuvants to the spray mixture.
Chemical weed control programs are seriously inadequate for control of nutsedges. Frequently, the weed germinates below the treated zone and avoids herbicide injury. Although many herbicides have been developed and tested in the last three decades, farmers still rely heavily on dinoseb, a herbicide developed in the 1950s. Dinoseb is a contact herbicide that causes injury to some field crops. Alternative approaches include monosodium methanearsonate (MSMA), paraquat, toxaphene, and triazine herbicides, but these chemicals are not registered for use on some crops or have been banned altogether in certain countries for reasons of toxicology and/or crop safety.
The use of chemical pesticides in agriculture is currently a major concern in the U.S. Nowhere is this concern more obvious than in the San Joaquin Valley of Calif., where pesticides are being blamed for an epidemic of cancer in children and young adults (Weisskopf, M. 1988! The Washington Post Weekly Edition 5(47):10-11, Washington, D.C.). New technologies in detection methods are enabling researchers to find pesticides in the environment that were previously thought to be totally degraded. One major public concern is protection of groundwater. The Environmental Protection Agency (EPA) estimates that 100,000 of the nation's 1.3 million wells are contaminated with pesticides (Fleming, M. H. 1987! Amer. J Alternative Agriculture 2:124-130). This has alarmed the general public since 50% of all Americans depend on groundwaterwells for their fresh water supplies. Because herbicides are so widely used in agriculture, and because they are often applied directly to the soil, the potential for movement into groundwater by leaching is perhaps greater than any other pesticide. Other inadequacies of chemical controls include lack of residual control, injury to non-target organisms, undesirable residues in harvested products, and carryover in subsequent crops.
There are no selective herbicides which can be used to control sedges in all crops. Indeed, those herbicides which are available do not always give acceptable control of these weeds. An alternative is offered by the use of microbes which have herbicidal activity specific for the problem weeds, and do not infect desirable plants.
Phatak et al. (Phatak, S. C., M. B. Callaway, C. S. Vavrina 1987! Weed Technology 1:84-91) have reviewed biological control of purple and yellow nutsedge. Cost effective procedures for the use of insects to control these species have not been developed. Phatak has reported that manipulation of a rust pathogen (Puccinia canaliculata) of yellow nutsedge has reduced stand, tuber formation, and completely inhibited flower formation of this weed. Phatak points out that little research has been directed toward integrating biological and chemical control of nutsedge. It was shown that rust-paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) combinations were much more effective than either treatment alone. Other work indicates that sequential applications of the rust and other herbicides, such as bentazon 3-(1 -methylethyl)-(1H)-2,1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide! provided significantly better control than either applied alone. Although this system is promising, there are many aspects that must be investigated, such as spore production and maintenance in storage. Phatak also states that other pathogens should be sought to complement the rust system.
Therefore, the use of bioherbicides is becoming an increasingly important alternative to chemical herbicides. This importance is exemplified by several patents which have been issued for bioherbicides and their use. Some of these patents, by way of illustration, are as follows: U.S. Pat. No. 3,849,104 (control of northern jointvetch with Colletotrichum gloeosporioides Penz. aeschynomene); U.S. Pat. No. 3,999,973 (control of prickly sida teaweed! and other weeds with Colletotrichum malvarum); U.S. Pat. No. 4,162,912 (control of milkweed vine with Araujia mosaic virus); U.S. Pat. No. 4,626,271 (cyanobacterin herbicide); U.S. Pat. No. 4,915,726 (biological control of dodder); U.S. Pat. No. 5,256,627 (control of nutsedge using Curvularia and Fusarium fungi).